Having been given confidence that I could find at least initial success in the Material Room by Lesson 3, this page will record my steps as I work more fully in that space.
My initial success was recorded on the main Lesson 3 page. There, I was able to select Katie's skirt and apply a texture to it. I was able to select Katie's shirt and apply a texture to it. What I wasn't able to do was change the texture, once applied. Perhaps this set of exercises will allow me to learn how.
If not, there's always George's Brute Force method.
Using the Material Room 101-Working With Image Maps tutorial, I created this simple-minded scene and applied Richard Dadd's Puck to the sphere.

The idea now is to apply something else to the ball — and have it render correctly.
In the Pose Room, I selected the ball and clicked into the Material Room. There, I made sure that I had selected the ball by checking under the Object menu. Yep, there it was. Just for good fortune, I checked the Material attached to the ball: there was only one, the Preview. With that set up, I clicked the image under the Diffuse Color column and selected John Singer Sargent's portrait of Caroline Bassano. It appeared in that image space, in the Material Preview space and — lo and behold! — in the Pose Room picture. I clicked back to the Pose Room and rendered. This is the result.

The next set of challenges was to determine what parts of the texture map corresponded to what parts of the image. So I put Sir Edmund Coley Burne-Jones's Heart of Briar Rose on his tshirt and began to play. In addition, I started to play with his pants (and I mean that in a nice way — no snickering). Here's the result.

This is the texture map I used for his shirt.

This is the texture map I used for his jeans

To prove to myself that I actually could do it, I tried this texture map with the jeans.

And got this:

As a designer, I don't think the folks during Fashion Week have anything to worry about. But I did notice that this appears to be a dynamic process. When I changed the file that Poser 6 looked at when it was time to select the material in the Material Room, the representation changed without my having to do anything!